1. Field of the Invention
This invention relates to cooling equipment for electronic systems, e.g., fans, and more particularly, to controlling the rotational speed of a fan.
2. Description of the Related Art
Fans are often used to evacuate warm air from enclosures in which electronic systems are contained. For example, most computer systems include one or more cooling fans to aid in circulating the air inside the enclosures and for maintaining the temperature inside the enclosures within an acceptable range. The increased airflow provided by fans typically aids in eliminating waste heat that may otherwise build up and adversely affect system operation. Employing cooling fans is especially helpful in ensuring proper operation for certain central processing units (CPUs), memory units, and other elements with relatively high operating temperatures.
Control of fans in a system typically involves a fan control unit executing a fan control algorithm. A fan control algorithm may determine the method for controlling one or more fans that are configured to evacuate warm air from a system enclosure. For example, the fan control algorithm may specify that a fan's speed should be increased or decreased dependent upon a detected temperature. Such control algorithms may also involve turning off a fan if the temperature is deemed cool enough to do so.
For detecting the temperature, a temperature sensor may provide to the fan control unit a signal indicative of the present temperature of a particular temperature zone in the electronic system. Fan drive systems often use a signal generator that provides a Pulse Width Modulated (PWM) signal to either directly drive the fan or drive an external circuit that controls the voltage between the power and ground interfaces of the fan, which in turn controls the speed of the fan. Signal generators that provide PWM signals are useful because they provide a digital control for the pulse width of a signal. The fan is typically powered only for the duration of the pulse. Between pulses power to the fan is turned off, although the fan is typically still spinning during this time. The duty cycle of the PWM pulse train presently being provided to the fan determines the fan's speed.
One problem in fan control has been to quiet the acoustical effects of a fan when the fan is changing speed. In general, present cooling solutions revolve around identifying and approaching a quiescent thermal operating point where the temperature and fan speed become stable. While in theory this may be considered an adequate and workable solution, in reality the fan speeds and zone temperatures typically do not reach this point, requiring the fan speed to be constantly readjusted, potentially increasing system noise. Consequently, most cooling solutions today typically attempt to minimize the change in PWM but generally don't prevent the PWM duty cycle from changing when the temperature begins to stabilize and is oscillating around the quiescent thermal operating point. In fact, even subtle changes in the PWM duty cycle may be perceived by the user.
Other corresponding issues related to the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.